10 mils

10 mils

40 mils

FR-4

0r ~ 4.5

Keep this

space free

from planes,

traces, pads,

and vias

Ground plane

Power plane

Low-speed traces

High-speed traces

27

ISO7340C, ISO7340FC, ISO7341C, ISO7341FC, ISO7342C, ISO7342FC

www.ti.com

SLLSEI6G – SEPTEMBER 2014 – REVISED JANUARY 2017

Product Folder Links: ISO7340C ISO7340FC ISO7341C ISO7341FC ISO7342C ISO7342FC

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Copyright © 2014–2017, Texas Instruments Incorporated

12 Layout

12.1 Layout Guidelines

A minimum of four layers is required to accomplish a low EMI PCB design (see Figure 28). Layer stacking should

be in the following order (top-to-bottom): high-speed signal layer, ground plane, power plane and low-frequency

signal layer.

•

Routing the high-speed traces on the top layer avoids the use of vias (and the introduction of their

inductances) and allows for clean interconnects between the isolator and the transmitter and receiver circuits

of the data link.

•

Placing a solid ground plane next to the high-speed signal layer establishes controlled impedance for

transmission line interconnects and provides an excellent low-inductance path for the return current flow.

•

Placing the power plane next to the ground plane creates additional high-frequency bypass capacitance of

approximately 100 pF/in2.

•

Routing the slower speed control signals on the bottom layer allows for greater flexibility as these signal links

usually have margin to tolerate discontinuities such as vias.

If an additional supply voltage plane or signal layer is needed, add a second power or ground plane system to

the stack to keep it symmetrical. This makes the stack mechanically stable and prevents it from warping. Also the

power and ground plane of each power system can be placed closer together, thus increasing the high-frequency

bypass capacitance significantly.

For detailed layout recommendations, see the Digital Isolator Design Guide.

12.1.1 PCB Material

For digital circuit boards operating at less than 150 Mbps, (or rise and fall times greater than 1 ns), and trace

lengths of up to 10 inches, use standard FR-4 UL94V-0 printed circuit board. This PCB is preferred over cheaper

alternatives because of lower dielectric losses at high frequencies, less moisture absorption, greater strength and

stiffness, and the self-extinguishing flammability-characteristics.

12.2 Layout Example

Figure 28. Recommended Layer Stack